p751: add tests for montgomery reduction implementations

p751toolbox/field_amd64_test.go

@@ -0,0 +1,73 @@
+// +build amd64,!noasm
+
+package p751toolbox
+
+import (
+	"golang.org/x/sys/cpu"
+	"testing"
+	"testing/quick"
+)
+
+func TestFp751MontgomeryReduce(t *testing.T) {
+	// First make sure that at least one value with a known result reduces
+	// correctly as defined in TestPrimeFieldElementToBigInt.
+	fp751MontgomeryReduce = fp751MontgomeryReduceFallback
+	t.Run("PrimeFieldElementToBigInt", TestPrimeFieldElementToBigInt)
+
+	if !cpu.X86.HasBMI2 {
+		return
+	}
+
+	fp751MontgomeryReduce = fp751MontgomeryReduceBMI2
+	t.Run("PrimeFieldElementToBigInt", TestPrimeFieldElementToBigInt)
+
+	// Also check that the BMI2 implementation produces the same results
+	// as the fallback implementation.
+	compareMontgomeryReduce := func(x, y PrimeFieldElement) bool {
+		var z, zbackup fp751X2
+		var zred1, zred2 Fp751Element
+
+		fp751Mul(&z, &x.A, &y.A)
+		zbackup = z
+
+		fp751MontgomeryReduceFallback(&zred1, &z)
+		// z may be destroyed.
+		z = zbackup
+		fp751MontgomeryReduceBMI2(&zred2, &z)
+
+		return zred1 == zred2
+	}
+
+	if err := quick.Check(compareMontgomeryReduce, quickCheckConfig); err != nil {
+		t.Error(err)
+	}
+
+	if !cpu.X86.HasADX {
+		return
+	}
+
+	fp751MontgomeryReduce = fp751MontgomeryReduceBMI2ADX
+	t.Run("PrimeFieldElementToBigInt", TestPrimeFieldElementToBigInt)
+
+	// Check that the BMI2ADX implementation produces the same results as
+	// the BMI2 implementation. By transitivity, it should also produce the
+	// same results as the fallback implementation.
+	compareMontgomeryReduce = func(x, y PrimeFieldElement) bool {
+		var z, zbackup fp751X2
+		var zred1, zred2 Fp751Element
+
+		fp751Mul(&z, &x.A, &y.A)
+		zbackup = z
+
+		fp751MontgomeryReduceBMI2(&zred1, &z)
+		// z may be destroyed.
+		z = zbackup
+		fp751MontgomeryReduceBMI2ADX(&zred2, &z)
+
+		return zred1 == zred2
+	}
+
+	if err := quick.Check(compareMontgomeryReduce, quickCheckConfig); err != nil {
+		t.Error(err)
+	}
+}